A. Continuing work using thermal ionization mass spectrometric (TIMS) analysis of calcium stable isotopic tracers for the measurement of fractional absorption from diet and the kinetics of whole body distribution have led to a number of clinically significant findings. 1) The mass of calcium in the rapidly exchanging internal pool (MPCa) has been determined from the intercept of the curve showing dilution of an intravenously administered tracer. This pool size has been shown to differ substantially from values expected on the basis of considering the pool to be a fixed fraction of body mass. The differences from the expected values in normal children (age range 2 wks - 14 yrs) correlate significantly (r=0.89, p<0.01, n=10) with incremental growth rate (cm/6 mos.). This suggests that the pool is an indicator of physiologically active bone mass or bone formation. Very preliminary data suggest that this pool is lower than expected in subjects with extensive bone demineralization as well. 2) The mean residence of calcium in the body, a measure of total body turnover, has been shown to relate directly to skeletal mass. This relationship has been shown to hold in normal humans over an age range of 2 wks - 45 yrs. 3) Studies of fractional absorption of dietary calcium in normal adult women have shown excellent agreement between radio and stable isotope tracer methodologies. B. TIMS has been used to measure magnesium tracer dilution in studies of bidirectional magnesium flux in barnacle fibers in vitro. Feasibility has been demonstrated for application of this technique to studies of flux changes undergone during perturbation of normal electrolyte concentrations in the muscle cell culture.